This invention relates to upgrading of data transmission resources in a telecommunications network by placing several signals on the same channel.
In telecommunications systems, the mobile stations and base transceiver stations may set up connections by way of the channels of a so-called radio interface. Demands on the data signalling rate, faultlessness and transfer delay are made on the connection depending on the type of information to be transferred.
A certain frequency range is always allocated for use by the mobile communications system. The frequency range is divided further into channels, whose data transfer resources are optimised according to the services provided by the telecommunications system. To have sufficient resources for use by the telecommunications system within the allocated limited frequency range, the channels in use must be used several times. For this reason, the coverage area of the system is divided into cells formed by the radio coverage areas of the individual base transceiver stations, and this is why the systems are often also called cellular radio systems.
Using the radio link, the mobile stations can use services provided by the telecommunications network. FIG. 1 shows the main structural features of a known telecommunications system. The network includes several interconnected MSCs (Mobile Services Switching Centres). The mobile services switching centre can set up connections with other mobile services switching centres or with other telecommunications networks, such as e.g. with ISDN (Integrated Services Digital Network), with PSTN (Public Switched Telephone Network), Internet, PDN (Packet Data Network), ATM (Asynchronous Transfer Mode) or with GPRS (General Packet Radio Service). Several base station controllers BSC are connected to the MSC. Base transceiver stations BTS are connected to each base station controller. The base transceiver stations may form connections with the mobile stations MS. A network management system NMS is used for collecting information from the network and for changing the programming of the network elements.
The air interface between base transceiver stations and mobile stations can be divided into channels in several different ways. Known methods are at least TDM (Time Division Multiplexing), FDM (Frequency Division Multiplexing) and CDM (Code Division Multiplexing). The band available in the TDM system is divided into successive time slots. A certain number of successive time slots form a periodically repeating time frame. The channel is determined by the time slot used in the time frame. In the FDM system the channel is determined by the frequency used, whereas in CDM systems the channel is determined by the used frequency hopping pattern or hash code. Combinations of the above-mentioned methods of division may also be used.
The information to be transferred is transported in a modulated form over the transmission channel. Known methods of modulation include amplitude modulation, where the information is contained in the signal amplitude, frequency modulation, where the information is contained in the signal frequency, and phase, modulation, where the information is contained in the signal phase. The transmission channel causes changes in the signal containing the information, so the signal perceived by the receiver is never an exact copy of the signal sent by the sender. In addition to amplitude attenuation, the sent signal will grow wider in the transmission channel, both on the frequency space and on the time space. Hereby the information contained in the signal by some modulation method will also change. The changes caused by the channel in the transmitted information can be corrected at the receiving end, if the channel characteristics are known with sufficient accuracy.
The maximum number of connections of the network servers and the data transmission resources of the connections indicate the resources of the telecommunications network. In a state-of-the-art system, one user signal can be conveyed on each channel. In addition, each channel has its own data transmission capacity. Under these circumstances, the system""s capacity or resources within the area of a certain cell is directly limited by the number of channels available in the cell.
The number of mobile station subscribers is growing strongly. At the same time, applications requiring much bandwidth, such as multimedia applications, are becoming more usual. In this situation, it is not possible,with state-of-the-art arrangements to efficiently utilise the available frequency spectrum without huge and expensive extension works in the network.
It is an objective of this invention to alleviate the above-mentioned problem by upgrading the resources of the telecommunications network This objective is achieved with the method and equipment described in the independent claims.
The inventive idea is in the same cell to send on the same channel several signals and with the receiver to cause different radio frequency characteristics for these. Thus a multiple quantity of information can be transferred on the same band. The different characteristics of signals are obtained e.g. by modulating the signals in different ways or by sending the signals from different points, whereby different signals will experience different radio channels. Based on the different characteristics, the signals can be distinguished from one another by joint detection or interference cancellation methods.
It is often advantageous to use different methods in the downlink direction from the base transceiver station to the mobile station and in the uplink direction from the mobile station to the base transceiver station.
According to one embodiment, different characteristics of different signals are brought about in the downlink direction by causing different phases for the signals to be transmitted. The mobile stations receiving the signals can hereby separate the signal addressed to themselves based on the different modulation of signals. It is not possible to synchronise that phase difference of signals sent in the uplink direction by mutually independent mobile stations which can be perceived by the base transceiver station""s antenna. Since the channels experienced by the signals are different, they may nevertheless be separated from one another at the base transceiver station with the aid e.g. of estimates of channel characteristics obtained with the aid of orthogonal training periods of the signals. However, since the signals are not entirely orthogonal, a considerable improvement of the signal quality is achieved by joint detection methods. Two or more users may thus use the same channel, whereby the system is able to serve more users.
In a second embodiment of the invention, at least one party to the data transmission, e.g. the base transceiver station, has several transmission antennas at its disposal. Hereby different radio channels experienced by signals transmitted from different antennas may be utilised. The mobile station or stations can distinguish from each other signals received from the same channel e.g. with the aid of such different channel characteristics experienced by the signals which they estimate with the aid of the training periods of the signals. If several signals to be transmitted to one mobile station are transmitted from different antennas, the data transmission rate of this mobile station is increased. If signals to be transmitted to different mobile stations are transmitted from different antennas, the number of users can be increased. However, since the channel transmission functions are superimposed, at least in part, it is advantageous to use the joint detection method.
The signals sent by mobile stations located at different points in the uplink traffic direction will automatically experience different radio channels. In additions a base transceiver station equipped with several antennas may hereby utilise interference elimination based on the antennas.